Work machines, mining equipment and other work vehicles may include a transmission coupled to a power source such as an internal combustion engine or an electric motor in order to provide more flexible use of the power output of the power source. The transmission may provide a number of gear ratios that enable the work machine to travel at a relatively wide range of speeds or conditions that might be impractical without a transmission. Some transmissions are configured to change gear ratios automatically in order to improve the ease of operation of the work machine as it is operated through its speed range.
The circumstances under which the transmission shifts gears may affect the efficiency of operation of the work machine. For example, the time at which the transmission shifts gears and the gears selected by the transmission may result in operating the power source at more efficient power source speeds and power outputs. For example, in general, it is more efficient to operate a power source at relatively lower power source speeds for a given power output. However, under certain conditions, it may be preferable to operate the power source in a range of power source speeds that result in relatively higher energy consumption at the expense of efficiency. If the machine is heavily loaded and/or travelling up a relatively steep or long grade, it may be preferable for the transmission to select gear ratios that provide improved performance even if efficiency may suffer.
In some work machine applications, it may be desirable for the transmission to provide directional shifting, or “shuttle shifting,” capability that permits the operator to command a machine direction reversal, and with the transmission responding by causing the work machine to slow down and then change direction. The directional shifting functionality alleviates the need for the operator to press a brake and stop the machine, move the transmission shifter from forward or drive to reverse or vice versa, and depress the accelerator. Transmissions providing directional shifting functionality are known in the art. For example, U.S. Pat. No. 5,353,662, issued to Vaughters on Oct. 11, 1994, entitled “Transmission Shuttle Shift Deceleration Method,” teaches a power shift transmission having a plurality of clutches, including a final or directional set of clutches. The clutches are controlled by electro-hydraulic direct acting valves and two electro-hydraulic proportional or pressure modulating valves. The pressure modulating valves modulate the pressure supplied to the direct acting valves associated with the three directional clutches. Vehicle deceleration during a shuttle shift is achieved by releasing or depressurizing all clutches, and then gradually pressurizing only two clutches in the final or directional set. The previously known transmissions performing directional shifting may only consider the speed of the vehicle and the current gear in determining the gear to engage in the reverse direction during a directional shift.
To perform a directional shift, an amount of energy is required to decelerate the machine in its current travel direction, and then accelerate the machine in the opposite direction. Similarly, energy is required to upshift, downshift or otherwise change the velocity of the machine. Much of the energy required for the change the machine's velocity is provided by the clutches within the transmission. As mentioned above, factors such as the loading on the machine and the grade up or down which the machine is traveling may be considered in the transmission control strategy to achieve a desired level of performance. These and other factors may also affect the amount of energy required to change velocity. For example, a loaded machine has more momentum than an unloaded machine, and consequently requires more energy to stop and then accelerate in the opposite direction. More energy is required to reverse the direction of a machine traveling downhill than the same machine traveling uphill. The increased energy requirement may cause increases in the heat generated at the transmission clutches during the velocity change, and can result in premature clutch failure when the clutch temperature repeatedly exceeds the material durability limits of the clutch components. However, such factors are not considered in previously known directional shift strategies. Similar issues can arise when the work machine accelerates and clutches cause the work machine to upshift and when the work machine decelerates and clutches maintain the current gear or downshift for engine braking. In view of this, opportunities exist for a gear selection strategy for upshifts, downshifts and high speed directional shifts that may prolong the useful life of the clutches and other components that can be subject to failure due to heat generated during the directional shifts.